Receiver or amplifier



Oct. 15, 1940. T. TILLMANN 2,218,101

RECEIVER 0R AMPLIFIER Filed Oct. 28. 1957 w J L5 INVENTOR THEOZ/LLMANN k? 7 WW ATTORNEY Patented Oct. 15, 1940 I Y UNITED STATES RECEIVER on AMPLIFIER 'Th'eo Tillmann, BerIimGei-many, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H. ,-Berlin, Germany, a corporation of Germany Application October 28, 1937, Serial No. 171,423 In Germany October 16,1936

3 Claims. (01. 179471) 1 In accordance with the invention the outer resistance of the end tube of a receiver, or amplifier is automatically increased at lower alternating grid voltage at the end tube or at lower 5 average alternating grid potential (for instance owing to volume control) said increase of the outer resistance being so high that the distortions at this lower alternating grid potential are smaller than in the absence ofsaid increase. In

this way it is accomplished that for each volume the quality of the reproduction is always the best possible one.

It is known that owing to non-linearity of the working characteristic, distortions appear in the 16 field of the grid voltage-plate current character'- istic of the end tube. These distortions depend on the outer resistance when assuming a definite type tube and fixed potentials, this dependence being such in a single grid tube, for in- 20 stance, that the distortions become smaller at a higher outer resistance. But at a higher outer resistance the range of control in the tube de-- creases, however, i. e., the power that can be furnished decreases. Hence, hitherto a compromise was necessary between the power that can be furnished and the allowable noise factor. The invention will be more readily understood from a consideration of the following detailed specification when read in conjunction with the drawing, wherein:

Figure 1 is a curve sheet used to explain certain characteristics of a power output tube operated according to the invention;

Figure 2 illustrates schematically'the embodiment of the invention providing adjustments of the transformation ratio of the output transformer; and.

Figure 3 is another embodiment of the invention using a variable impedance device to adjust 40 the output impedance. 1

These conditions become clear from Figure 1. The characteristics show the dependence of the value of the noise factor on the power when considering outer resistances differing in value. In case of a low outer resistance according to the characteristic I, the noise factor increases greatly at an increase in power (this being shown in exaggeration for the sake of clearness) but the said noise factor is already high at the beginning. In case of a higher outer resistance for instance in accordance with the characteristic 4, the high increase occurs already at lower power, so that the range of control is very much smaller. But within this range the noise factor on the contrary is low.

The invention utilizes this condition in that at'high alternating grid potential the operation is carried out with a low outer resistance (for instance in accordance with characteristic I) and at lower alternating grid potential, with a 5 higher outer resistance (for instance, according to characteristic 4). At a less extended' control (alternating grid potential) and at the same outer resistance the noise factor becomes, however, already smaller as seen from the course 10 taken by the characteristics, but when using the invention the noise factor for the respective control is reduced to the lowest possible value. It should be borne in mind, however, that in the v case of a single grid tube the volume decreases 15 when the outer resistance is increased, wherefore, at a decrease of the volume the alternating grid potential of the end tube can not be decreased to such an extent as is possible without the use of the invention. However an 20 optimum of. small noise factor can be achieved by a suitable relation between the control curves for the alternating grid potential and. outer resistance.

. trol of the volume such that the outer resistance will be increased when the volume control means at the grid of the end tube, for instance, is controlled downwards.

Another possibility residesin that according to 40 Figure 3 a resistor is placed in series to the loudspeaker coil, and which when reducing the volume will be switched in and then increased, whereas at full volume only the loudspeaker coil is in the circuit.

It is also within the scope of the invention that the outer resistance of the end tube automatically sets itself in dependence of the respective amplitude, while in the above examples the dependence is on the average volume taken over 5 a larger period. This would, for instance, be possible in that with the detected audio-frequency the conduction of a resistor placed ahead of the loudspeaker coil would be varied, for instance,

by preliminary magnetizing of a .choke. 5:;

I claim:

1. In an audio frequency current amplifier provided with an electronic tube having a normal direct current component of plate current and provided with an input circuit coupled to a source of signal energy and an output circuit coupled to an external load impedance, the method of controlling the output of the amplifier without altering the direct current resistance of the external plate circuit whereby said normal direct current component of plate current is maintained, which consists in adjusting the amount of coupling between the source and the input of the amplifier, and simultaneously varying the value of the external load impedance in such a direction that the external load impedance is increased as the amount of coupling between the source and the amplifier input is decreased.

2. In signaling apparatus, an electronic tubev having a normal direct current component of plate current and provided With an input circuit coupled to a source of signal energy and an output circuit coupled to an external load impedance,

means for controlling the output of said amplifier without altering the direct current resistance of the external plate circuit whereby said normal direct current component of plate current is maintained comprising means for simultaneously varying the amount of coupling between the source and an input circuit of the amplifier and the value of the external load impedance in such a direction that the external load impedance is increased as the amount of coupling between the source and the amplifier input circuit is decreased.

3. The arrangement described in claim 2 characterized by that the output circuit is coupled to the external load impedance through a transformer and by that the value of the external load impedance is varied by varying the ratio of transformation of said transformer, the arrangement being such that the ratio of transformation is decreased as the coupling between the source and the input circuit is decreased.

THEO TILLMANN. 

